riscv: Remove MMU dependency from Zbb and Zicboz

The Zbb and Zicboz ISA extensions have no dependency on an MMU and are
equally useful on NOMMU kernels.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240227003630.3634533-4-samuel.holland@sifive.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>